Multiple antenna energy harvesting

ABSTRACT

Energy harvesting circuits and associated methods are provided that employ multiple antennas to optimize the amount of energy that is harvested while at the same time making efficient use of tag space. In some embodiments, matching networks are chosen in a manner that optimizes the DC energy that is created from the harvesting process. In other embodiments, phase shifts are introduced into the received signals to allow the signals to be more efficiently combined after they are rectified.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.11/619,786, entitled “Multiple Antenna Energy Harvesting,” which wasfiled on Jan. 4, 2007, now U.S. Pat. No. 7,528,698 which applicationclaims the benefit of U.S. Provisional Application No. 60/756,309,entitled “Multiple Antenna Energy Harvesting,” which was filed on Jan.5, 2006, the disclosures of which are incorporated herein by reference.

FIELD OF THE INVENTION

The present invention relates to energy harvesting circuits forharvesting RF energy transmitted through the air and converting it to DCenergy for powering electronic devices such as an RFID tag ortransponder.

BACKGROUND OF THE INVENTION

The harvesting of RF energy transmitted through the air for use inpowering electronic devices is extremely important in a number offields, such as radio frequency identification (RFID) systems, securitymonitoring and remote sensing, among others. For example, RFID systemsconsist of a number of radio frequency tags or transponders (RFID tags)and one or more radio frequency readers or interrogators (RFID readers).The RFID tags typically include an integrated circuit (IC) chip, such asa complementary metal oxide semiconductor (CMOS) chip, and an antennaconnected thereto for allowing the RFID tag to communicate with an RFIDreader over an air interface by way of RF signals. In a typical RFIDsystem, one or more RFID readers query the RFID tags for informationstored on them, which can be, for example, identification numbers, userwritten data, or sensed data.

RFID tags can generally be categorized as either passive tags or activetags. Passive RFID tags do not have an internal power supply. Instead,the electrical current induced in the antenna of a passive RFID tag bythe incoming RF signal from the RFID reader provides enough power forthe IC chip or chips in the tag to power up and transmit a response. Onepassive tag technology, known as backscatter technology, generatessignals by backscattering the carrier signal sent from the RFID reader.In another technology, described in U.S. Pat. Nos. 6,289,237 and6,615,074, RF energy from the RFID reader is converted to a DC voltageby an antenna/matching circuit/charge pump combination. The DC voltageis then used to power a processor/transmitter/antenna combination thattransmits information to the RFID reader at, for example, a differentfrequency. In either case, the area of the tag or silicon die isvaluable, and therefore it is advantageous to make the most efficientuse of the space thereon. In addition, it is known that multipleantennas can be used to generate a DC voltage from an RF signal. Forexample, U.S. Pat. No. 6,734,797 describes a tag that uses two dipoleantennas where the greater of the energies produced from the twoantennas is the one that is selected and used. This, however, is not themost efficient use of tag space since the energy for the “loser” antennais not used. Thus, there is a need for an energy harvesting circuit thatis able to employ multiple antennas to optimize the amount of energythat is harvested while at the same time making efficient use of tagspace.

SUMMARY OF THE INVENTION

In one embodiment, an energy harvesting circuit, and associated method,is provided that includes a plurality of antennas, wherein each of theantennas is tuned to the same particular RF frequency range. Each of theantennas is structured to receive an RF signal having the sameparticular RF frequency and output a respective AC signal. The circuitalso includes a plurality of matching networks, wherein each of thematching networks is operatively coupled to a respective one of theantennas and is structured to receive the AC signal output by therespective one of the antennas. The circuit further includes a pluralityof voltage boosting and rectifying circuits, wherein each of the voltageboosting and rectifying circuits is operatively coupled to a respectiveone of the matching networks and is structured to receive the AC signalreceived by the respective one of the matching networks and output a DCvoltage signal by converting the received AC signal into the DC voltagesignal. In addition, the DC voltage signals output by the voltageboosting and rectifying circuits are summed together to create acombined DC voltage signal, and the impedance of each of the matchingnetworks is chosen in a manner so as to maximize the voltage level ofthe DC voltage signal that is output by the associated voltage boostingand rectifying circuit.

In another embodiment, an energy harvesting circuit, and associatedmethod, is provided that includes a plurality of antennas provided in anantenna plane, wherein each of the antennas is tuned the same particularRF frequency range. Each of the antennas is structured to receive an RFsignal having the same particular RF frequency range and output arespective AC signal. The circuit also includes a plurality of matchingnetworks, wherein each of the matching networks is operatively coupledto a respective one of the antennas and is structured to receive the ACsignal output by the respective one of the antennas. The circuit furtherincludes a plurality of voltage boosting and rectifying circuits,wherein each of the voltage boosting and rectifying circuits isoperatively coupled to a respective one of the matching networks and isstructured to receive the AC signal received by the respective one ofthe matching networks and output a DC voltage signal by converting thereceived AC signal into the DC voltage signal. In addition, the DCvoltage signals output by the voltage boosting and rectifying circuitsare summed together to create a combined DC voltage signal, and theimpedance of each of the matching networks is chosen so as to cause avoltage level of the combined DC voltage signal to have a minimumdeviation as a function of angle of rotation as the antenna plane isrotated about a first axis. Alternatively, the impedance of each of thematching networks may be chosen so as to cause a voltage level of thecombined DC voltage signal to have at least a predetermined minimumvalue as the antenna plane is rotated about a first axis. As still afurther alternative, the impedance of each of the matching networks maybe chosen by: (i) incrementally rotating the antenna plane through aplurality of angle increments about a first axis, (ii) trying aplurality of different inductance and capacitance value combinations foreach LC tank circuit of each of the matching networks at each of theangle increments, (iii) measuring the combined DC voltage signal foreach of the inductance and capacitance value combinations at each of theangle increments, (iv) determining which one of the inductance andcapacitance value combinations produces a maximum voltage level for themeasured combined DC voltage signals at any one of the angle increments;(v) choosing for each the LC tank circuit the one of the inductance andcapacitance value combinations that produces the maximum voltage level.

In yet another embodiment, an energy harvesting circuit, and associatedmethod, is provided that includes a plurality of antennas, wherein eachof the antennas is tuned to the same particular RF frequency range, andwherein each of the antennas is structured to receive an RF signalhaving the particular RF frequency range and output a respective ACsignal. The circuit further includes phase shifting and rectifyingcircuitry operatively coupled to the antennas. The phase shifting andrectifying circuitry is structured to: (i) receive each respective ACsignal, (ii) create a plurality of out of phase AC signals by causingeach respective AC signal to be out of phase with one another, and (iii)convert each of the out of phase AC signals into a respective DC voltagesignal. The DC voltage signals are then summed together to create acombined DC voltage signal.

Therefore, it should now be apparent that the invention substantiallyachieves all the above aspects and advantages. Additional aspects andadvantages of the invention will be set forth in the description thatfollows, and in part will be obvious from the description, or may belearned by practice of the invention. Moreover, the aspects andadvantages of the invention may be realized and obtained by means of theinstrumentalities and combinations particularly pointed out in theappended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings illustrate presently preferred embodiments ofthe invention, and together with the general description given above andthe detailed description given below, serve to explain the principles ofthe invention. As shown throughout the drawings, like reference numeralsdesignate like or corresponding parts.

FIG. 1 is a block diagram of one embodiment of an energy harvestingcircuit according to the present invention;

FIG. 2 is a schematic illustration of an embodiment of a square spiralantenna that may be used in the energy harvesting circuits describedherein;

FIG. 3 is a schematic illustration of an embodiment of an antenna layoutthat may be used in the energy harvesting circuits described herein;

FIG. 4 is a Smith chart that may be employed to choose the matchingnetworks as described herein;

FIG. 5 is a schematic illustration of an energy harvesting circuit asdescribed herein positioned within the range of a suitable RF source;

FIG. 6 is a block diagram of an energy harvesting circuit according toalternative embodiment of the invention in which a phase shift isintroduced into each of the received RF signals; and

FIGS. 7, 8 and 9 are schematic illustrations of three particularembodiments of the energy harvesting circuit shown in FIG. 6.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 is a block diagram of one embodiment of an energy harvestingcircuit 5 according to the present invention. The energy harvestingcircuit 5 may, for example, form part of an RF transponder, such as anRFID tag, for providing power thereto or form part of some other type ofremote station for energizing a power storage device thereof or someother object of interest. The energy harvesting circuit 5 includes aplurality of antennas 10A-10D, each of which is tuned to the sameparticular RF frequency range. As used herein, the term “RF frequencyrange” shall refer to a single RF frequency or a band of RF frequencies.As is known, each antenna 10A-10D may have a tank circuit (not shown)that includes an inductor and a capacitor, wherein the inductance andcapacitance values are chosen to tune the antenna 10A-10D to the desiredRF frequency range, i.e., the same particular RF frequency or band of RFfrequencies. In this embodiment, each tank circuit is fixed (i.e., fixedinductor and capacitor) so that the antenna 10A, 10B, 10C, 10D isfixedly tuned to the RF frequency range of interest.

While four antennas 10A-10D are shown in FIG. 1, it should be understoodthat that is meant to be exemplary only, and that the plurality ofantennas may include less than or more than four antennas (with eachsuch antenna being operatively coupled to respective accompanyingcircuitry as described below). Furthermore, each antenna 10A, 10B, 10C,10D may be, for example, a square spiral antenna 35 having the formshown in FIG. 2, with the innermost end 40 being open and the outermostend 45 being connected to a matching network (matching network 15A, 15B,15C, 15D shown in FIG. 1) as described below. In this manner, the squarespiral antenna 35 may be viewed as an asymmetric dipole, and thus thesquare spiral antenna 35 does not need a ground plane. Preferably, thelength of the outermost segment 50 of the square spiral antenna 35 isabout equal to a quarter of the wavelength of the particular RFfrequency or center frequency of the band of RF frequencies to which itis tuned as described herein, and the total length of the square spiralantenna 35 (three segments) is about equal to one half of thatwavelength.

FIG. 3 is a schematic illustration of one particular antenna layout thatmay be used in the energy harvesting circuit 5 shown in FIG. 1 thatincludes four antennas 35 positioned and spaced in the manner shown withthe outermost end 45 of each antenna 35 being connected to therespective matching network 15A, 15B, 15C, 15D as described below. Ithas been observed that the conductor width and spacing of the antennas35 in FIG. 3 can be varied without seriously degrading the performancemeasured as the DC output of the respective charge pumps 20A, 20B, 20C,20D described below. Therefore, in the layout shown in FIG. 3, it ispossible to put four antennas 35 in an area that is slightly more thantwo times the area occupied by single antenna 35.

Referring again to FIG. 1, each antenna 10A, 10B, 10C, 10D isoperatively coupled to a respective matching network 15A, 15B, 15C, 15Dand charge pump 20A, 20B, 20C, 20D as shown in FIG. 1. In particular,each antenna 10A, 10B, 10C, 10D is electrically connected to arespective matching network 15A, 15B, 15C, 15D, which in turn iselectrically connected to a respective voltage boosting and rectifyingcircuit preferably in the form of a one or more stage charge pump 20A,20B, 20C, 20D. Charge pumps are well known in the art. Basically, onestage of a charge pump essentially doubles the effective amplitude of anAC input voltage and stores the resulting increased DC voltage on anoutput capacitor. The voltage could also be stored using a rechargeablebattery. Successive stages of a charge pump, if present, willessentially increase the voltage from the previous stage resulting in anincreased output voltage. In operation, each antenna 10A, 10B, 10C, 10Dreceives energy, such as RF energy, that is transmitted in space by afar-field source, such as an RF source. The RF source may be, forexample, an RF interrogator unit which transmits an RF interrogationsignal, or a local radio station, in which case the RF energy comprisesambient RF energy in the vicinity of the energy harvesting circuit 5.The RF energy received by each antenna 10A, 10B, 10C, 10D is provided,in the form of an AC signal, to the associated charge pump 20A, 20B,20C, 20D through the associated matching network 15A, 15B, 15C, 15D.Each charge pump 20A, 20B, 20C, 20D rectifies the received AC signal toproduce a respective DC signal 25A, 25B, 25C, 25D, wherein each DCsignal 25A, 25B, 25C, 25D is amplified as compared to what it would havebeen had a simple rectifier been used. The individual DC signals 25A,25B, 25C, 25D are then summed to produce a combined DC signal 30, whichmay be used to power a circuit or charge a power storage device asdescribed elsewhere herein. The individual DC signals 25A, 25B, 25C, 25Dmay be summed in any suitable manner, such as, without limitation,connecting them in parallel or connecting them in series to produce thecombined DC signal 30.

In one particular embodiment, each matching network 15A, 15B, 15C, 15Dis chosen (i.e., its impedance is chosen) so as to maximize the outputof the respective charge pump 20A, 20B, 20C, 20D in the form of therespective DC signal 25A, 25B, 25C, 25D (i.e., to maximize the DC signal25A, 25B, 25C, 25D). In other words, each matching network 15A, 15B,15C, 15D is matching the impedance of the respective antenna 10A, 10B,10C, 10D to the respective charge pump 20A, 20B, 20C, 20D solely on thebasis of maximizing the DC output of the respective charge pump 20A,20B, 20C, 20D. In the preferred embodiment, each matching network 15A,15B, 15C, 15D is an LC circuit of either an L topology (which includesone inductor and one capacitor) or a π topology (which includes oneinductor and two capacitors) wherein the inductance of the LC circuitand the capacitance of the LC circuit are chosen so as to maximize theDC output of the respective charge pump 20A, 20B, 20C, 20D. In oneembodiment, an LC tank circuit may be formed by the inherent distributedinductance and inherent distributed capacitance of the conducingelements of each antenna 10A, 10B, 10C, 10D, in which case the antennais designed and laid out in a manner that results in the appropriatechosen L and C values.

In the preferred embodiment, each matching network 15A, 15B, 15C, 15D ischosen so as to maximize the output of the respective charge pump 20A,20B, 20C, 20D using a trial and error (“annealing”) empirical approach.In particular, various sets of inductor and capacitor values are used asmatching elements in each matching network 15A, 15B, 15C, 15D, and theresulting output of the respective charge pump 20A, 20B, 20C, 20D ismeasured for each combination, and the combination that produces themaximum output is chosen. In this process, the input impedance of thecharge pump (20A, 20B, 20C, 20D) with each matching network (15A, 15B,15C, 15D) may be plotted as a point on a Smith chart with a color codingfor the amount of energy harvested as shown in FIG. 4. After a number oftries, it is easy to see a clustering of the color coded points toselectively choose other points in or around the cluster to achieve anear optimum value. FIG. 4 illustrates a number of points showing theresults on a Smith chart. It is important to note that the matchingachieved in this manner tends to optimize the asymmetric dipoleconfiguration described elsewhere herein. The matching component valuesare indicated in FIG. 4 as circles on the Smith chart with grayscaleindications of voltage at the charge pump (20A, 20B, 20C, 20D) output ascompared to the 0 to 5 volt grayscale on the right of FIG. 4.

In this embodiment, the matching network (15A, 15B, 15C, 15D) is chosenindividually for each antenna (10A, 10B, 10C, 10D) in the energyharvesting circuit 5 in the manner just described. Other embodiments arealso possible and are described below.

As will be appreciated by those of skill in the art, each antenna 10A,10B, 10C, 10D that is included in the energy harvesting circuit 5, forexample as laid out in FIG. 3, will have a different polarization withrespect to the transmitting antenna of the RF source that is determinedby the relative angle of the antenna 10A, 10B, 10C, 10D in question tothe polarization of the transmitting antenna of the RF source. Anotherembodiment of the invention, described below, chooses each matchingnetwork (15A, 15B, 15C, 15D) in a manner whereby the relativepolarization efficiency of each antenna 10A, 10B, 10C, 10D will be moreefficiently utilized. In particular, in this embodiment, the energyharvesting circuit 5 is set up as shown in FIG. 1 with the antennas 10A,10B, 10C, 10D being laid out as desired, for example in the manner shownin FIG. 3. Then, as shown in FIG. 5, the energy harvesting circuit 5 ispositioned within the range of a suitable RF source 60, preferably of atype identical or similar to the one with which the energy harvestingcircuit 5 will ultimately be used, with the plane 70 in which theantennas are laid out 10A, 10B, 10C, 10D being substantiallyperpendicular to a line 65 that is taken through the center of thetransmitting antenna of the RF source 60. While the RF source 60 istransmitting, the antenna is incrementally rotated about the line 65,preferably through 360 degrees (the rotation will be into and out of thepaper in FIG. 5). At each increment (each angle of rotation θ), thematching elements (the inductance and capacitance values) in eachmatching network 15A, 15B, 15C, 15D are varied over a set of differentvalues (preferably predetermined), and with each such variation, thevoltage level of the combined DC signal 30 is measured and recorded. Thesame set of inductance and capacitance values is used at each angle θ.In addition, at each angle θ, the set of values may and preferably willinclude instances where the values are different for one or more of thematching networks, i.e., each matching network will not have the samevalues. Once the rotation is complete, the measured and recorded levelsof the combined DC signal 30 (as a function of the angle θ) are analyzedand the set of inductance and capacitance values yielding the minimumdeviation in the combined DC signal 30 as a function of the angle θ ischosen. The matching networks 15A, 15B, 15C, 15D are then structuredaccordingly (i.e., to have the chosen inductance and capacitance values)in the final energy harvesting circuit 5. As will be appreciated, thiswill result in the combined DC signal 30 having the minimum deviation inall orientations of the energy harvesting circuit 5 in the plane inwhich the antennas are laid out. As a result, at least some minimumamount of DC voltage will be able to be harvested in all orientations ofthe energy harvesting circuit 5. This may be important if, for example,the circuit or object (or some component thereof) to be powered requiressome minimum voltage level to operate. The matching networks 15A, 15B,15C, 15D can be chosen as described above to ensure that at least thatminimum voltage level is produced in all orientations (i.e., all anglesof rotation θ) of the energy harvesting circuit 5. In addition, once thematching networks 15A, 15B, 15C, 15D (i.e., the inductance andcapacitance values therefor) are chosen in this manner (which may betime consuming) for one energy harvesting circuit 5, multiple energyharvesting circuits 5 that meet the same parameters can be mass producedwithout having to go through the same trial and error steps.

In another embodiment, once the rotation is complete, the measured andrecorded levels of the combined DC signal 30 are analyzed and the set ofinductance and capacitance values yielding a maximum level for thecombined DC signal 30 at any one orientation is chosen. The matchingnetworks are then structured accordingly (i.e., to have the choseninductance and capacitance values) in the final energy harvestingcircuit 5.

In the energy harvesting circuit 5 shown in FIG. 1, it will beappreciated that each of the AC signals provided to the matchingnetworks 15A, 15B, 15C, 15D will be in phase with one another. As aresult, the individual antenna, matching, charge pump, etc. circuitshave an RF supply in which the individual RF sinusoidal signals are inphase. This is analogous to the single phase rectifier design which iswell known in DC rectifier circuitry. The removal of the minimum valuesof DC output is strictly on the basis of the angular variation of theantenna circuitry through matching the angles of orientation, and notthe angles of the individual sinusoidal sources. Thus, in the energyharvesting circuit 5 shown in FIG. 1 the signals are not combined in themost efficient manner, and a more efficient manner is described below inconnection with FIG. 6.

FIG. 6 is a block diagram of an energy harvesting circuit 75 accordingto alternative embodiment of the invention in which a phase shift isintroduced into each of the received RF signals. In particular, theenergy harvesting circuit 75 includes a plurality of antennas 80A-80C(similar to the antennas 10A-10D), each of which is fixedly tuned to thesame particular RF range. While three antennas 80A-80C are shown in FIG.6, it should be understood that that is meant to be exemplary only, andthat the plurality of antennas may include less than or more than threeantennas (with each such antenna being operatively coupled to respectiveaccompanying circuitry as described below). Furthermore, each antenna80A, 80B, 80C may be, for example, a square spiral antenna 35 having theform shown in FIG. 2. In the case of four antennas, the layout may be asshown in FIG. 3.

As shown in FIG. 6, each antenna 80A, 80B, 80C is electrically connectedto respective phase shifting circuitry 85A, 85B, 85C, which in turn iselectrically connected to a respective voltage boosting and rectifyingcircuit preferably in the form of a one or more stage charge pump 95A,95B, 95C. In operation, each antenna 80A, 80B, 80C receives energy, suchas RF energy, that is transmitted in space by a far-field source, suchas an RF source. The RF source may be, for example, an RF interrogatorunit which transmits an RF interrogation signal, or a local radiostation, in which case the RF energy comprises ambient RF energy in thevicinity of the energy harvesting circuit 75. The RF energy received byeach antenna 80A, 80B, 80C is provided, in the form of an AC signal, tothe associated phase shifting circuitry 85A, 85B, 85C. Each phaseshifting circuitry 85A, 85B, 85C shifts the phase of the received ACsignal in a manner such that the AC signals 90A, 90B, 90C output by thephase shifting circuitry 85A, 85B, 85C are all out of phase with oneanother. Thus, each phase shifting circuitry 85A, 85B, 85C introduces adifferent degree of phase shift (which may actually be zero, in whichcase that particular phase shifting circuitry may be omitted, or,alternatively, be arranged to simply not shift phase or shift phase by360 degrees). In one embodiment, the phase shift introduced by eachphase shifting circuitry 85A, 85B, 85C results in each AC signal 90A,90B, 90C being out of phase from the immediately adjacent signal ACsignal 90A, 90B, 90C by an equal amount. For example, for three ACsignals 90A, 90B, 90C, there would be 120 degrees separating immediatelyadjacent signals (e.g., one with 0 degrees phase shift, one with 120degrees phase shift and one with 240 degrees phase shift, or,alternatively, one with 20 degrees phase shift, one with 140 degreesphase shift and one with 260 degrees phase shift).

The AC signal AC signal 90A, 90B, 90C output by each phase shiftingcircuitry 85A, 85B, 85C is provided to the associated charge pump 95A,95B, 95C. In an alternative embodiment, the AC signal AC signal 90A,90B, 90C output by each phase shifting circuitry 85A, 85B, 85C may beprovided to the associated charge pump 95A, 95B, 95C through anassociated matching network chosen and configured in the mannersdescribed elsewhere herein in order to optimize DC output. In eitherembodiment, each charge pump 95A, 95B, 95C amplifies and rectifies thereceived AC signal to produce a respective DC signal 100A, 100B, 100C.The individual DC signals 100A, 100B, 100C are then summed (as describedelsewhere herein) to produce a combined DC signal 105, which may be usedto power a circuit or charge a power storage device as describedelsewhere herein. Because the AC signal 90A, 90B, 90C received by thecharge pumps 95A, 95B, 95C are all out of phase with one another, theyare able to be converted to DC and summed to produce the combined DCsignal 105 in a more efficient manner, thereby leading to an increase onthe voltage of the combined DC signal 105 over what would have been thecase had the phase shifts described herein not been introduced. This isthe case because the phase shifts cause the peaks and valleys of the ACsignals 90A, 90B, 90C be offset form one another rather then lined upwith one another. It will also be appreciated that the multiphaseembodiment provides a second degree of freedom in design for rotationalpurposes.

FIG. 7 is schematic illustration of one particular embodiment of theenergy harvesting circuit 75, labeled as 75′. The energy harvestingcircuit 75′ includes antennas 80A, 80B, 80C as described above, each ofwhich is fixedly tuned to the same particular RF frequency range andoutputs an AC signal. Each antenna 80A, 80B, 80C is connected to phaseshifting and rectifying circuitry 110 that performs the functionality ofthe phase shifting circuitry 85A, 85B, 85C and the charge pumps 95A,95B, 95C to produce the respective DC signals 100A, 100B, 100C. Asdescribed elsewhere herein, the DC signals 100A, 100B, 100C are thensummed (as described elsewhere herein) to produce a combined DC signal105. The phase shifting and rectifying circuitry 110 includes a wellknown Cockroft-Walton multiplier and consists of inductors 112, 114, and116, capacitors 118, 120, 122, 124, 126, and 128, and diodes 130, 132,134, 1365, 138, 140, 142, 144 and 146 connected as shown in FIG. 7. Thephase shifting and rectifying circuitry 110 receives each AC signal fromthe antennas 80A, 80B, 80C, shifts the phases thereof such that each ACsignal is out of phase with one another, and boosts and rectifies theresulting signals to produce the DC signals 100A, 100B, 100C. The valuesof the inductors 112, 114, and 116 and the capacitors 118, 120, and 122determines the degree of phase shift applies to each respective ACsignal received from the antennas 80A, 80B, 80C. FIG. 8 shows a similarembodiment of an energy harvesting circuit 75″ where two antennas areused with similar phase shifting and rectifying circuitry 110′. It willbe appreciated that four or more antennas may be used in a similarmanner. FIG. 9 shows yet another similar embodiment of an energyharvesting circuit 75′″ where two antennas are used with similar phaseshifting and rectifying circuitry 110″.

While preferred embodiments of the invention have been described andillustrated above, it should be understood that these are exemplary ofthe invention and are not to be considered as limiting. Additions,deletions, substitutions, and other modifications can be made withoutdeparting from the spirit or scope of the present invention.Accordingly, the invention is not to be considered as limited by theforegoing description but is only limited by the scope of the appendedclaims.

1. An energy harvesting circuit, comprising: a plurality of antennas,each of said antennas being tuned to the same particular RF frequencyrange, each of said antennas being structured to receive an RF signalhaving said particular RF frequency range and output a respective ACsignal; a plurality of matching networks, each of said matching networksbeing operatively coupled to a respective one of said antennas and beingstructured to receive the AC signal output by said respective one ofsaid antennas; and a plurality of voltage boosting and rectifyingcircuits, each of said voltage boosting and rectifying circuits beingoperatively coupled to a respective one of said matching networks andbeing structured to receive the AC signal received by the respective oneof said matching networks and output a DC voltage signal by convertingthe received AC signal into the DC voltage signal; wherein the DCvoltage signals output by the voltage boosting and rectifying circuitsare summed together to create a combined DC voltage signal, wherein eachof said matching networks is an LC tank circuit and wherein an impedanceof each of the matching networks is chosen in manner so as to maximize avoltage level of the DC voltage signal that is output by the associatedone of the voltage boosting and rectifying circuits by, for each LC tankcircuit of each of the matching networks: (i) trying a plurality ofdifferent inductance and capacitance value combinations for the LC tankcircuit, (ii) measuring the voltage level of the DC voltage signal thatis output by the associated one of the voltage boosting and rectifyingcircuits for each of the inductance and capacitance value combinations,and (iii) choosing one of the inductance and capacitance valuecombinations that produces a maximum voltage level of the DC voltagesignal that is output by the associated one of the voltage boosting andrectifying circuits.
 2. The energy harvesting circuit according to claim1, wherein each of said voltage boosting and rectifying circuits is acharge pump.
 3. The energy harvesting circuit according to claim 1,wherein each of said antennas is a square spiral antenna having anoutermost segment, a middle segment and an innermost segment, wherein alength of each outermost segment of is equal to about a quarter of thewavelength of the particular RF frequency or center frequency of theband of RF frequencies included in said particular RF frequency range,wherein a total length of each square spiral antenna is equal to aboutone half of said wavelength, and wherein the outermost segment of eachof said antennas is operatively coupled to the associated one of saidmatching networks.
 4. The energy harvesting circuit according to claim3, wherein each square spiral antenna alone occupies a first spatialarea, and wherein said antennas are provided in an antenna layout thatoccupies a second spatial area that is less than two times said firstspatial area.
 5. An energy harvesting circuit, comprising: a pluralityof antennas, each of said antennas being tuned to the same particular RFfrequency range, each of said antennas being structured to receive an RFsignal having said particular RF frequency range and output a respectiveAC signal; a plurality of matching networks, each of said matchingnetworks being operatively coupled to a respective one of said antennasand being structured to receive the AC signal output by said respectiveone of said antennas; and a plurality of voltage boosting and rectifyingcircuits, each of said voltage boosting and rectifying circuits beingoperatively coupled to a respective one of said matching networks andbeing structured to receive the AC signal received by the respective oneof said matching networks and output a DC voltage signal by convertingthe received AC signal into the DC voltage signal; wherein the DCvoltage signals output by the voltage boosting and rectifying circuitsare summed together to create a combined DC voltage signal, and whereinan impedance of each of the matching networks is chosen in manner so asto maximize a voltage level of the DC voltage signal that is output bythe associated one of the voltage boosting and rectifying circuits; andwherein each of said antennas is a square spiral antenna having anoutermost segment, a middle segment and an innermost segment, wherein alength of each outermost segment of is equal to about a quarter of thewavelength of the particular RF frequency or center frequency of theband of RF frequencies included in said particular RF frequency range,wherein a total length of each square spiral antenna is equal to aboutone half of said wavelength, and wherein the outermost segment of eachof said antennas is operatively coupled to the associated one of saidmatching networks.
 6. The energy harvesting circuit according to claim5, wherein each of said voltage boosting and rectifying circuits is acharge pump.
 7. The energy harvesting circuit according to claim 5,wherein each of said matching networks is an LC tank circuit.
 8. Theenergy harvesting circuit according to claim 5, wherein each squarespiral antenna alone occupies a first spatial area, and wherein saidantennas are provided in an antenna layout that occupies a secondspatial area that is less than two times said first spatial area.